Conventional collecting and condensing designs for electromagnetic radiation emphasize collecting and redirecting the maximum amount of light from a source of radiation, approximated by a point source. To produce a small spot size based on these designs results in a decrease in radiation flux because conventional designs (which focus on the collection and redirection of the maximum amount of light) inherently conflict with the goal of concentrating the radiation flux into the smallest possible spot size when the radiation originates from conventional incoherent sources. Thus, images having small spot sizes may be obtained only with a corresponding decrease in flux density.
There are two basic optical designs in common use for collecting and condensing radiation. The first is a system of condenser lenses such as illustrated in FIG. 1. Condenser lenses have several problems which include the creation of chromatic and spherical aberrations and the large amount of space which is required for the setup. Ellipsoidal reflectors (as shown in FIG. 2a) are also used in prior art systems. As used in the prior art, the source of electromagnetic radiation is placed at the primary focal point, and the target (e.g., a fiber optic bundle) is placed at the secondary focal point with the fiber axis, 18, parallel to the major axis, 12, of the ellipsoid. Both primary and secondary focal points are collinear with the optical axis which is identical to the major axis. Such a system would be described as an "on-axis" system and has a number of disadvantages including high cost; higher than desirable magnification of the image, resulting in a reduction of flux density collectable by a small target such as a fiber optic; shadowing of radiation reducing the total collectable flux; and incomplete utilization of the surface of the reflector. On-axis ellipsoidal reflecting systems tend to emphasize redirection of the maximum amount of flux from a point source at the expense of the flux density, as discussed above.
U.S. Pat. No. 4,757,431 (FIG. 3a) describes an improved condensing and collecting system employing an "off-axis" spherical concave reflector which increases the amount of flux illuminating and collectable by a small target. The off-axis spherical concave reflector described in this patent has certain disadvantages: (i) the presence of optical aberrations and astigmatism parallel to the direction of the off-axis displacement and (ii) physical limitations inherent in the requirement to minimize the off-axis distance. The effect of astigmatism is to decrease the concentrating efficiency of the system and thereby reduce the flux collected at the target. The requirement to minimize the off-axis distance between the source and the target (so as to minimize the astigmatic distortion), imposes limitations on the physical dimensions of a source and target of the described embodiment. Improvements in the collection efficiency of the off-axis system can be made by substituting for the spherical reflector a toroidal reflector (FIG. 3b) which reduces the astigmatism and optical aberrations inherent in the off-axis configuration. However, inherent limitations in a toroidal off-axis system prevent maximum collection efficiency.
The invention described below is an "off-axis" ellipsoidal condensing and collecting system of radiation. In comparing the two systems shown in FIG. 2b and FIG. 2a, some major differences between the off-axis and the on-axis systems include the following with respective to the off-axis 14 system: (i) the optical axis of the target will always be situated at an angle greater than zero degrees to the major axis 21 of the ellipsoid along which are located source S and target T separated by a source-target distance, ST, and (ii) the optical or geometric axis, 12, of the reflecting portion, P, of the ellipsoidal reflector M1 will not coincide with the major axis 21 by contrast to the on-axis system of FIG. 2a in which both the major axis and the geometric or optical axis of the ellipsoid are coincident. Compared to the prior art "off-axis" condensing and collecting system, FIG. 3a, the present invention is a more exact imaging system of unit magnification which preserves the brightness of the source at the target and therefore provides substantial improvements in the coupling of radiation between a source and a target.